This invention relates to novel methods and apparatus for providing a plurality of functionally integrated subsurface logging measurements. More particularly, this invention relates to techniques for providing pluralities of logging measurements each of which is integrated on the basis of borehole depth.
If is often necessary, in the search for subsurface oil and gas-bearing earth formations, to survey or "log" the formations by passing a logging sonde through a borehole which measures several parameters at various depths within the borehole. This information is thereafter processed, analyzed and compared to similar data generated at correlative depths in accordance with known functional relationships to determine whether the formation is of commercial interest.
A serious problem with such comparison of data is that frequently only comparison or "matching" of data points generated at the same borehole depth or time, or generated at a preselected correlative different depth or time, will yield meaningful results. However, because measurements are often made during two or more passes through the borehole, if is often difficult to insure that measurements from each pass selected for comparison were made at the same relative depths or times.
One reason for this is that depth indications at which measurements are made have been historically unreliable, being dependent on several factors which include differences in downhole assemblies, borehole fluids and logging cables to name but a few.
Moreover, the conventional depth measuring method of measuring passage of the logging cable over a measuring wheel introduces still further variation in recorded depth measurements. For example, dimensional variations of the measuring wheel due to temperature and wear, as well as oscillation of the measuring wheel from varying cable tension contribute to the depth measurement variations. Still further, logging cable slippage past the measuring wheel and logging cable stretch due to temperature and humidity effects, the weight of the cable and logging tool, and tension due to borehole strictures further compound the problem of obtaining accurate depth indications.
When a parameter or logging curve generated during one logging operation is to be compared with an identical parameter measured at an earlier time within the same borehole, difficulties are encountered due to the hereinabove noted problems which render the matching of data points derived at the same depths extremely difficult. However, the correlation of data points becomes even more difficult when the identical parameters are not being compared, due to the dissimilarity in the logging curve appearances and values. Still further, it is sometimes necessary to compare measurements derived from correlative depths within different boreholes, and again, even when the measurements are with respect to identical parameters, such comparison is often rendered extremely difficult. Moreover even though measurements may be derived as a function of depth in response to a depth-dependent command signal sent downhole as disclosed in the hereinbefore noted copending U.S. patent application Ser. No. 949,592, the above noted factors may still contribute to apparent discrepancies in depth at which measurements from two or more logs are made.
Various techniques have been used in the past to correlate two or more sets of well logging data points. One such technique involves the side-by-side visual comparison of conventional film-type logging records of two or more parameters, whereby logging curves on one such film may be shifted with respect to the other, and correlative measurements thereafter read and recorded from the film. This attempted solution has numerous disadvantages, such as the inability to make such correlations in real-time as the log to be correlated is generated, the need for conventional film-type log records, and the fact that the end product of such a correlation is not in a form suitable for further conventional processing.
Another attempted solution to the problem of providing correlated logging measurements has been to store each set of logging data by an appropriate means, such as digital tape recordings, and thereafter selectively retrieving matched data pairs derived at similar depths for further processing. One obvious problem associated with this method is that, as with the use of graphical light table approaches, all data to be correlated is first generated and recorded. Thus the possibility of real-time processing of data correlated as it is generated and the real-time recording of such correlated data is precluded.
Yet another problem with this approach is that frequently no sample from the second set of data is available which was derived at the identical borehole depth as the sample from the first set of data. Thus, it may be necessary to interpolate between data points from the second set to estimate a sample value which will correlate with a sample from the first set with respect to the depth at which they were derived.
Thus, it should be readily apparent that it would be desirable to provide a method and apparatus whereby any logging parameters of interest may be insured of being measured at correlative borehole depths during successive passes of the sonde through the same boreholes or different as any other such measurements previously derived, so as to increase their usefulness for data accuracy and reliability checks, for combining such measurements derived at correlative borehole depths to determine functional relationships, and the like. Accordingly, it would also be highly desirable to adjust the depth at which future logging measurements are generated or sampled in response to either visual or automatic analysis of the presently generated samples either alone or in comparison with other historically derived data. Moreover, it would also be desirable to sample or select on a depth basis historically derived samples for correlative matching with respect to depth with samples from other data sets by a visual or automatic comparison of other sample points from each of the data sets. These and other features are provided in the present invention which also overcomes the hereinbefore noted serious problems associated with the prior art, wherein novel techniques and apparatus are provided for correlating two or more sets of logging data in a manner to be hereinafter more particularly set forth.